This invention relates generally to the field of testing systems and methods, and more particularly to systems and methods for transport or permeation testing. In one particular aspect, the invention provides systems and methods for culturing cells onto a membrane and then using the membrane to simulate an epithelial cell layer, such as the cells which form the inner lining of a human intestine or blood vessel. In this way, transport or permeation tests may be performed using the membrane.
In humans, ingested food passes from the stomach to the small intestine where proteins, fats, carbohydrates and other nutrients are absorbed and distributed into circulation for use in various organs and cells throughout the body. The small intestine is about five to six meters in length and has an extremely large surface area for absorbing nutrients and other materials. The interior of the small intestine includes the mucosal epithelium which comprises small fingerlike projections called villi which protrude into the intestine and provide the nutrient absorption surface.
For a variety of reasons, it is desirable to study and evaluate how various chemicals which are orally ingested into a human will be absorbed into the blood stream through the intestinal wall. Such evaluation can be useful in, for example, drug testing to determine how various drugs will be absorbed into the blood stream. Transport of various substances through other types of epithelial cells can also be useful in therapeutically treating patients.
In order to evaluate how certain chemicals or substances will permeate epithelial cells, some have proposed growing mammalian-based cells on a membrane which in turn is used to mimic a cell layer within the body. To properly culture the cells on the membrane, it is usually best to have the membrane horizontally oriented when seeding the cells on the membrane. Hence, some previously proposed testing systems comprise a cup having a membrane at its bottom end. In this way, cells may be seeded on the membrane while the membrane is horizontally oriented. After the cells have grown onto the membrane, the cup is inserted into a larger well and various chemicals are placed into the cup to evaluate how the chemicals will permeate the cells on the membrane and into fluid in the larger well.
Such testing systems suffer from a variety of drawbacks, including limited access to both sides of the membrane, particularly since the bottom side of the membrane will be enclosed by the well into which the cup is inserted. Another drawback is the significant amount of time required to separately seed the cells into each of the walls. A further drawback to such systems is their limited use in accommodating smaller sized membranes. For example, many multi-well plates are being provided with increased numbers of wells whose dimensions are significantly smaller to create larger densities of wells within the plates. Accordingly, each membrane needs to be made smaller in order to fit within the smaller wells. However, when reducing the size of the membranes with the testing systems described above, the membrane's surface area may be too small to provide an adequate transport interface. In turn, this can lower concentrations or transported amounts to levels which restrict analytical methodologies presently available to quantify results. Further, the activity provided by a cell layer on such small membrane sizes may not be representative of the activity provided by a cell layer on a larger membrane.
Hence, for these and other reasons, it would be desirable to provide systems and methods which will allow cells to be seeded horizontally in an efficient manner. In some cases it would also be desirable to allow access to both sides of the membrane during a testing procedure. Further, it would be desirable to provide a design for a testing system where membrane densities are greatly increased while still being sufficiently sized to effectively accommodate cell growth and to provide an adequate transport interface.